1. Field of the Invention
The invention relates to portable radiotelephones, and more particularly, to portable radiotelephones having an antenna that is extendable and/or selectively connectable to external test equipment.
2. Description of Related Art
Mobile dispatch radio systems provide two-way radio communication between a central control station and a fleet of cars, trucks, or other vehicles. Typical users of dispatch systems include, for example, police departments, fire departments, taxi services and delivery services. Present dispatch systems can be configured to provide radio communication between the control station and all vehicles in a fleet, between the control station and selected vehicles in the fleet, or between different individual vehicles or groups of vehicles within a fleet. Transmission trunked systems group a number of channels together for a particular coverage zone, enabling two or more users to communicate at the same time. Each channel is comprised of a pair of assigned frequencies, a mobile transmit frequency and a repeater transmit frequency, with the repeater transmit frequencies typically being either 39 MHz or 45 MHz above a corresponding mobile transmit frequency. Dispatch systems are typically simplex systems; the control station and the mobile units, for example, cannot transmit on the same channel simultaneously.
Dispatch radios have a transmission power sufficient to reach a repeater associated with the control station from anywhere within a particular coverage zone that is relatively large, on the order of 100 square miles. Typical dispatch radios have higher transmission powers than cellular portable radios, for example, and typically use a substantial metallic plate as part of the radio housing in order to maximise transmit power while grounding the radio and reducing transmission background noise to an acceptable level. A substantial metallic plate also provides better heat sink and ruggedness. Additionally, dispatch radios typically use fixed antennas, as opposed to extendable antennas. Users of dispatch systems typically believe that fixed antennas are necessary to provide the transmission characteristics required in dispatch environments, and that extendable antennas are not sufficiently rugged and durable in such environments. Typical fixed antennas have an impedance of 50 ohms and are connected to the dispatch radio by a 50 ohm antenna connector.
Unlike dispatch systems, typical cellular radiotelephone systems are duplex systems that allow simultaneous transmission by two mobile transceivers on a particular channel, comprised of a receive/transmit pair of frequencies, for point-to-point communication. Coverage zones in cellular systems are relatively small compared to dispatch coverage zones, on the order of five square miles, and a network of small coverage zones blankets the area served by the cellular system.
Because cellular coverage zones are relatively small, cellular radios require less transmission power than dispatch radios; typical cellular transmission power is three watts or less, and often less than one watt. Typical cellular radios do not use the substantial metallic plate present in typical dispatch radios. Typical cellular radio housings are often formed entirely of plastic, and contain small amounts of metal, in a PC board, for example, for heat sink purposes. Users of cellular systems generally have preferred extendable antennas because they provide greater compactness, easier storage and a more desirable appearance. Additionally, because extendable antennas can be withdrawn into the radio, extendable antennas are less likely to be broken off or otherwise damaged during periods of non-use. Typical extendable antennas have an impedance of 600 ohms and are connected to cellular radios by a 600 ohm antenna connector.
Attempts to use extendable antennas in radios with substantial mettalic plates, such as typical dispatch radios, have been unsuccessful. As described above, dispatch radios typically use 50 ohm antenna connectors, and extendable antennas have an impedance of 600 ohms. Using a 600 ohm antenna connection point in radios with a substantial metallic plate generates significant stray capacitance at the connection point between the antenna and the metallic plate of the radio, preventing the radio from functioning properly. Additionally, the large amount of metal in the plate tends to detune an extendable antenna. Although typical cellular radios have plastic housings and thus avoid the problems associated with metallic plates, radios with high power requirements typically use metallic plates as part of the housing and thus typically do not use extendable antennas.
With both cellular and dispatch radios, it is often desirable to use external test equipment, such as signal generators, power meters, deviation monitors, frequency counters, etc., to determine whether the radios are functioning properly. Typical test equipment has an impedance of 50 ohms and thus requires a 50 ohm connection point to the radio. To facilitate such connection, many cellular radios using 600 ohm antenna connectors for 600 ohm extendable antennas include a 50 ohm test port in the housing for the radio, separate from the 600 ohm extendable antenna connector, for receiving a coaxial cable that connects to the test equipment. A switch mechanism typically is provided within such radios, for alternately connecting either the antenna connector or the test port to the radio circuit. Mechanical wear and other problems, however, render such switch mechanisms subject to failure. Additionally, the expense and complexity of such mechanisms are quite disadvantageous.
The above reasons have generally precluded the design of a single portable radio housing that would service either a dispatch radio or a cellular radiotelephone. Accordingly, it would be desirable to provide a universal portable radio housing having antenna and test connector ports and capable of supporting both dispatch and cellular radios. It also would be advantageous to provide a dispatch radio having an antenna connector accommodating a 600 ohm extendable antenna, to achieve the advantages of extendable antennas in a dispatch environment. It also would be advantageous to eliminate the extra test ports typically found on cellular radios, to avoid the problems associated with such test ports.